Low optical loss electrode structures for LEDs

ABSTRACT

An electrode structure is disclosed for enhancing the brightness and/or efficiency of an LED. The electrode structure can have a metal electrode and an optically transmissive thick dielectric material formed intermediate the electrode and a light emitting semiconductor material. The electrode and the thick dielectric cooperate to reflect light from the semiconductor material back into the semiconductor so as to enhance the likelihood of the light ultimately being transmitted from the semiconductor material. Such LED can have enhanced utility and can be suitable for uses such as general illumination.

TECHNICAL FIELD

The present invention relates generally to light emitting diodes (LEDs).The present invention relates more particularly to electrode structuresthat mitigate optical losses and thus tend to enhance the brightnessand/or the efficiency of LEDs.

BACKGROUND

Light emitting diodes (LEDs) for use as indicators are well known. LEDshave been used extensively for this purpose in consumer electronics. Forexample, red LEDs are commonly used to indicate that power has beenapplied to such devices as radios, televisions, video recorders (VCRs),and the like.

Although such contemporary LEDs have proven generally suitable for theirintended purposes, they possess inherent deficiencies that detract fromtheir overall effectiveness and desirability. For example, the lightoutput of such contemporary LEDs is not as great as is sometimesdesired. This limits the ability of contemporary LEDs to function insome applications, such as providing general illumination, e.g., ambientlighting. Even high power contemporary LEDs do not provide sufficientillumination for such purposes.

At least a part of this problem of insufficient brightness is due toinefficiency of contemporary LEDs. Efficiency of LEDs is a measure ofthe amount of light provided as compared to the electrical powerconsumed. Contemporary LEDs are not as efficient as they can be becausesome of the light generated thereby is lost due to internal absorption.Such internal absorption limits the amount of light that can beextracted from an LED and thus undesirably reduces the efficiencythereof.

Thus, although contemporary LEDs have proven generally suitable fortheir intended purposes, they possess inherent deficiencies whichdetract from their overall effectiveness and desirability. As such, itis desirable to provide LEDs that have enhanced brightness and/orefficiency.

BRIEF SUMMARY

Systems and methods are disclosed herein to provide brighter and/or moreefficient LEDs. For example, in accordance with an embodiment of thepresent invention, an LED can comprise a reflective electrode structurecomprising a metal electrode.

More particularly, the electrode can be formed upon a semiconductormaterial that emits light having a central wavelength λ. This light isemitted in all directions. A comparatively thick, optically transmissivedielectric material can be formed upon the semiconductor material. Aportion of the electrode can be formed over the comparatively thickdielectric material. Another portion of the same electrode can be inelectric contact with the semiconductor material. The electrodecooperates with the thick dielectric to enhance reflection such thatlight emitted in the direction of the electrode is reflected back intothe semiconductor material and thus has another opportunity to beextracted from the LED.

The term wavelength (λ), as used herein, refers to the wavelength oflight inside of the material that the light is traveling within. Thus,if light within a semiconductor material is being referred to, forexample, then the wavelength of this light is its wavelength within thesemiconductor material.

The thick dielectric thickness can be greater than ½λ, where λ is theWavelength of light inside of the thick dielectric material. The thickdielectric material can have an index of refraction that is lower thanthat of the semiconductor material and that is greater than or equal 1.0The light emitting semiconductor material can comprise AlGaAs, AlInGaP,AlInGaN, and/or GaAsP, for example. Other materials can similarly besuitable.

The optically transmissive thick dielectric layer can be a comparativelythick layer of material such as silicon dioxide, silicon monoxide, MgF₂and siloxane polymers, and/or air, for example. Other materials cansimilarly be suitable.

There can be an ohmic contact layer between the metal electrode and thesemiconductor. The ohmic contact layer can comprise indium tin oxide(ITO), nickel oxide, and/or RuO₂, for example. Other materials cansimilarly be suitable. The ohmic contact layer can be part of thesemiconductor device comprising of a heavily doped layer.

There can be a current spreading layer between the metal electrode andthe semiconductor. The current spreading layer is composed of indium tinoxide, nickel oxide, RuO₂, for example. Other materials can similarly besuitable.

A series of one or more pairs of DBR dielectric layers can be formedbetween the thick dielectric layer and the metal electrode such thateach DBR dielectric layer of this pair can be optically transmissive, ofdifferent indices of refraction from each other, and/or odd multiples ofabout ¼λ thick.

Each layer of the pairs of DBR dielectric material can comprise titaniumdioxide TiO₂, Ti₃O5, Ti₂O₃, TiO, ZrO₂, TiO₂ZrO₂Nb₂O₅, CeO₂, ZnS, Al2O₃,SiN niobium pentoxide (Nb₂O₅), tantalum pentoxide (Ta₂O₅), siloxanepolymers SiO, SiO₂, and/or MgF₂, for example. Other materials cansimilarly be suitable.

The metal electrode can be comprise one or more metal layers, whereineach metal layer can be selected from a group consisting of Al, Ag, Rh,Pd, Cu, Au, Cr, Ti, Pt nickel/gold alloys, chrome/gold alloys,silver/aluminum mixtures and combinations thereof. Other materials cansimilarly be suitable.

The LED can have either a vertical or lateral structure. A portion ofthe metal electrode can form an area for wire bonding. A portion of themetal electrode can make an electrical contact to the semiconductormaterial at the edges of the thick dielectric material. A portion of themetal electrode makes an electrical contact to the semiconductormaterial through openings in the thick dielectric material.

According to one embodiment of the present invention, a reflectiveelectrode structure for an LED comprises a metal electrode. A GaNmaterial emits light about some central wavelength λ. A comparativelythick silicon dioxide material can be formed upon the GaN material. Aportion of the electrode can be formed over the thick dielectricmaterial. Another portion of the same electrode can be in ohmic contactwith a semiconductor material. The thick dielectric can have a thicknessgreater than ½λ. Both the dielectric material and the metal electrodecan make physical contact to the semiconductor via an ITO layer or othermaterials than can be similarly suitable.

According to one embodiment of the present invention, a reflectiveelectrode structure comprises a metal electrode and a GaN material emitslight about some central wavelength λ. A thick silicon dioxide materialcan be formed upon the GaN material. A series of at least one DBR paircan be formed upon the thick silicon dioxide material.

A portion of the electrode can be formed over both the thick dielectricmaterial and the DBR pairs. Another portion of the same electrode can bein ohmic contact with the semiconductor material. The thick dielectricthickness can be greater than ½λ.

Each layer of the DBR pairs can be optically transmissive, of differentindices of refraction with respect to one another, and can be oddmultiples of about ¼λ in thickness. Both the thick dielectric and themetal electrode can make physical contact to the semiconductor via anITO layer.

Thus, according to one or more embodiments of the present invention abrighter and/or more efficient LED can be provided. Increasing thebrightness and/or efficiency of LED enhances their utility by makingthem more suitable for a wider range of uses, including generalillumination.

This invention will be more fully understood in conjunction with thefollowing detailed description taken together with the followingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing the concept of critical angle;

FIG. 2 is a semi-schematic perspective view of a contemporary lateralLED structure;

FIG. 3 is a semi-schematic perspective view of a contemporary verticalLED structure;

FIG. 4A is a semi-schematic diagram showing light reflection at acontemporary GaN/Cr/Au interface;

FIG. 4B is a chart showing reflectivity at the GaN/Cr/Au interface ofFIG. 4A for different angles of incidence;

FIG. 5A is a semi-schematic diagram showing a contemporary electrodestructure having an ohmic contact layer;

FIG. 5B is a semi-schematic diagram showing a contemporary electrodestructure having an ohmic contact/current spreading layer;

FIG. 6A is a semi-schematic top view of a portion of a contemporary LEDdie showing a circular contact that can also function as a bond pad;

FIG. 6B is a semi-schematic top view of a portion of a contemporary LEDdice showing a cross shaped contact with a circular bond pad;

FIG. 6C is a semi-schematic top view of a portion of a contemporary LEDdice showing exemplary contact geometry that is suitable for use withlarger LEDs and having a circular contact that can also function as abonding pad;

FIG. 7A is a semi-schematic side view of a contemporary low aspect ratioelectrode structure;

FIG. 7B is a semi-schematic side view of a high aspect ratio electrodestructure;

FIG. 8A is a semi-schematic diagram showing light reflection at acontemporary Ag interface;

FIG. 8B is a chart showing reflectivity at the Ag interface of FIG. 8Afor different angles of incidence;

FIG. 9A is a semi-schematic diagram showing light reflection at acontemporary GaN/SiO₂/Ag interface of a vertical structure LED;

FIG. 9B is a chart showing reflectivity at the GaN/SiO₂/Ag interface ofFIG. 9A for different angles of incidence;

FIG. 10A is a semi-schematic diagram showing light reflection at acontemporary GaN/air interface;

FIG. 10B is a chart showing reflectivity at the GaN/air interface ofFIG. 10A for different angles of incidence;

FIG. 11A is a semi-schematic diagram showing light reflection at aGaN/SiO₂ interface, wherein the thick dielectric is thick according toan embodiment of the present invention;

FIG. 11B is a chart showing reflectivity at the GaN/SiO₂ interface ofFIG. 11 A for different angles of incidence;

FIG. 12A is a semi-schematic diagram showing light reflection at aGaN/SiO₂/Al interface according to an embodiment of the presentinvention;

FIG. 12B is a chart showing reflectivity at the GaN/SiO₂/Al interface ofFIG. 12A for different angles of incidence wherein thicknesses of theSiO₂ layer are less than or equal to 1¾ the wavelength of incident lightaccording to an embodiment of the present invention;

FIG. 12C is a chart showing reflectivity at the GaN/SiO₂/Al interface ofFIG. 12A for different angles of incidence wherein thicknesses of theSiO₂ layer are greater than 1¾ the wavelength of incident lightaccording to an embodiment of the present invention;

FIG. 13A is a semi-schematic diagram showing light reflection at adistributed Bragg reflector (DBR) comprised of alternating layers ofSiO₂ and TiO₂ according to an embodiment of the present invention;

FIG. 13B is a chart showing reflectivity at the DBR layers of FIG. 13Afor different angles of incidence according to an embodiment of thepresent invention;

FIG. 14 is a chart showing reflectivity of several materials fordifferent angles of incidence according to an embodiment of the presentinvention;

FIG. 15A is a semi-schematic diagram showing a first exemplaryembodiment of a suspended electrode according to the present invention;

FIG. 15B is a semi-schematic diagram showing a second exemplaryembodiment of a suspended electrode according to the present invention;

FIG. 15C is a semi-schematic diagram showing a third exemplaryembodiment of a suspended electrode according to the present invention;

FIG. 15D is a semi-schematic diagram showing a fourth exemplaryembodiment of a suspended electrode according to the present invention;

FIG. 15E is a semi-schematic diagram showing a fifth exemplaryembodiment of a suspended electrode according to the present invention;

FIG. 15F is a semi-schematic diagram showing a sixth exemplaryembodiment of a suspended electrode according to the present invention;

FIG. 16A is a semi-schematic diagram showing a first exemplaryembodiment of a suspended electrode with an ohmic contact layeraccording to the present invention;

FIG. 16B is a semi-schematic diagram showing a second exemplaryembodiment of a suspended electrode with an ohmic contact layeraccording to the present invention;

FIG. 16C is a semi-schematic diagram showing a third exemplaryembodiment of a suspended electrode with an ohmic contact layeraccording to the present invention;

FIG. 16D is a semi-schematic diagram showing a fourth exemplaryembodiment of a suspended electrode with an ohmic contact layeraccording to the present invention;

FIG. 16E is a semi-schematic diagram showing a fifth exemplaryembodiment of a suspended electrode with an ohmic contact layeraccording to the present invention;

FIG. 16F is a semi-schematic diagram showing a sixth exemplaryembodiment of a suspended electrode with an ohmic contact layeraccording to the present invention;

FIG. 17A is cross-section view of a contemporary lateral structure LED;

FIGS. 17B-17D are semi-schematic perspective views showing some steps inthe process for fabricating the LED of FIG. 17A;

FIG. 18A is cross-section view of a lateral structure LED according toan embodiment of the present invention;

FIGS. 18B-18E are semi-schematic perspective views showing some steps inthe process for fabricating the LED of FIG. 18A;

FIG. 19A is cross-section view of a lateral structure LED according toan embodiment of the present invention;

FIGS. 19B-17E are semi-schematic perspective views showing some steps inthe process for fabricating the LED of FIG. 19A;

FIG. 20A is a semi-schematic perspective view showing another embodimentof suspended structure according to an embodiment of the presentinvention;

FIG. 20B is a semi-schematic perspective view showing another embodimentof suspended structure according to an embodiment of the presentinvention;

FIG. 21A is a semi-schematic diagram showing an exemplary embodiment ofthe present invention in an elongated LED;

FIG. 21B is a semi-schematic diagram showing an exemplary embodiment ofthe present invention in an elongated LED;

FIG. 22A is a semi-schematic diagram showing an exemplary embodiment ofthe present invention in an elongated LED;

FIG. 22B is a semi-schematic diagram showing an exemplary embodiment ofthe present invention in an elongated LED;

FIG. 22C is a semi-schematic diagram showing an exemplary embodiment ofthe present invention in an elongated LED;

FIG. 23A is a semi-schematic diagram showing an exemplary embodiment ofthe present invention in an elongated LED;

FIG. 23B is a semi-schematic diagram showing an exemplary embodiment ofthe present invention in an elongated LED;

FIG. 23C is a semi-schematic diagram showing an exemplary embodiment ofthe present invention in an elongated LED; and

FIG. 24 is a semi-schematic diagram showing an exemplary embodiment ofthe present invention in an elongated LED.

Embodiments of the present invention and their advantages are bestunderstood by referring to the detailed description that follows. Itshould be appreciated that like reference numerals are used to identifylike elements illustrated in one or more of the figures.

DETAILED DESCRIPTION

Light emitting devices (LEDs) emit light in response to excitation by anelectrical current. One typical LED has a heterostructure grown on asubstrate by metal-organic vapor phase epitaxy or a similar technique.An LED heterostructure includes n-type and p-type semiconductor layersthat sandwich a light producing layer, i.e., an active region. Exemplaryactive areas may be quantum wells surrounded by barrier layers.Typically, electrical contacts are attached to the n-type and p-typesemiconductor layers. When a forward bias is applied across theelectrical contacts electrons and holes flow from n-type and p-typelayers to produce light in the active region. Light is producedaccording to well known principles when these electrons and holesrecombine with each other in the active region.

The efficiency with which a LED converts electricity to light isdetermined by the product of the internal quantum efficiency, thelight-extraction efficiency, and losses due to electrical resistance.The internal quantum efficiency is determined by the quality of thesemiconductor layers and the energy band structure of the device. Bothof these are determined during deposition of the semiconductor layers.

The light extraction efficiency is the ratio of the light that leavesthe LED chip to the light that is generated within the active layers.The light extraction efficiency is determined by the geometry of theLED, self-absorption of light in semiconductor layers, light absorptionby electrical contacts, and light absorption by materials in contactwith the LED that are used to mount a device in a package.

Semiconductor layers tend to have relatively high indices of refraction.Consequently, most of the light that is generated in the active regionof an LED is internally-reflected by surfaces of a chip many timesbefore it escapes. To achieve high light-extraction efficiency it isimportant to minimize absorption of light by the semiconductor layersand by electrical connections to the chip. When these layers are made tohave very low optical absorption, by being transparent or highlyreflective, the overall light extraction in an LED is improvedsubstantially.

Referring now to FIG. 1, light inside of a high index of refractionmedium 11 is incident at interface to a lower index of refraction medium12. The light can be incident at different angles. When light from ahigh index of refraction medium 11 encounters the interface to a lowerindex of refraction medium 12 the light can either be transmitted intothe lower index of refraction medium 12 or be reflected back into thehigher index of refraction medium 11.

According to Snell's law, a portion of the light traveling from amaterial having an index of refraction n₁ into a material having a lowerindex of refraction n₂ at an angle less than the critical angle Θ_(c)will pass into the lower index of refraction material. This is indicatedby the arrow on the left that continues from the material having thelower index of refraction n₁ into the material having the higher indexof refraction n₂.

Conversely, according to Snell's law, all of the light traveling from amaterial having a higher index of refraction n₁ toward a material havinga lower index of refraction n₂ at an angle greater than the criticalangle Θ_(c) will be reflected back into the higher index of refractionmaterial. This mechanism is know is total internal reflection (TIR) andis indicated by the arrow on the right that does not continue from thematerial having the higher index of refraction n₁ into the materialhaving the lower index of refraction but the arrow rather extends backthrough the material having the higher index of refraction.

Light within a material having a higher index of refraction than existsoutside of the material (such as light within a semiconductor materialwhere air or an encapsulating epoxy is the outside material) which isincident upon the interface surface at angles greater than Θ_(c) willexperience total internal reflection. Typical semiconductor materialshave a high index of refraction compared to ambient air (which has anindex of refraction of 1.0), or encapsulating epoxy (which can have anindex of refraction of approximately 1.5).

In an LED, this light is reflected back into the LED chip where furtherabsorption can undesirably occur from other materials. This undesirableabsorption reduces the efficiency of the LED by reducing the amount oflight that the LED provides.

For conventional LEDs, the vast majority of light generated within thestructure suffers total internal reflection before escaping from asemiconductor chip. In the case of conventional Gallium Nitride (GaN)based LEDs on sapphire substrates, about 70% of emitted light can betrapped between the sapphire substrate and the outer surface of the GaN.This light is repeatedly reflected due to total internal reflection,thus suffering multiple absorptions by the metal electrodes and theother materials. It is thus desirable to create structures that tend tominimize this absorption.

As used herein, the term electrode can refer to a conductor (such as ametal conductor) that supplies current to a semiconductor material of anLED. Thus, an electrode can be in electrical contact with thesemiconductor material. However, not all portions of an electrode arenecessarily in contact with the semiconductor material. Indeed,according to one or more embodiments of the present invention, a portionof an electrode is in electrical contact with the semiconductor materialand another portion of an electrode is not in electrical contact withthe semiconductor.

Referring now to FIG. 2, a contemporary lateral structure LED is shown.Regions on the surface of a p-layer 21 and an n-layer 22 of an LED 20can be metallized so as to form electrodes 23 and 24. p-n junction oractive region 26 is between p-layer 21 and an n-layer 22. Electrodes 23and 24 provide a means to provide electrical power to LED 20. For devicestructures where the semiconductor is supported by an opticallytransparent, electrically non-conductive substrate 23, comprised of amaterial such as sapphire, the electrical contact to p-layer 21 andn-layer 22 must be made from the top surface.

In the configuration shown in FIG. 2, p-layer 21 is already exposed attop surface and electrical contact can be readily made therewith.However n-layer 22 is buried beneath both p-layer 21 and active region26. To make electrical contact to n-layer 22, a cutout area 28 is formedby removing a portion of p-layer 21 and active layer 26 (the removedportion is indicated by the dashed lines) so as to expose n-layer 24therebeneath. After the creation of cutout area 28, the n-layerelectrical contact or electrode 24 can be formed.

Such device structures as that shown in FIG. 2 result in the currentflowing generally in the lateral direction. This is why they arereferred to as lateral structures. One disadvantage of such lateralstructures is that a portion of the active light producing region mustbe removed to produce the cutout structure 28 so the n-electrode 24 canbe formed. Of course, this reduces the active region area andconsequently reduces the ability of LED 20 to produce light.

Referring now to FIG. 3, an LED 30 can alternatively comprise structureswhere the semiconductor (comprised of a p-layer 31 and an n-layer 32that cooperate to define an active region 36) is supported by anelectrically conductive substrate 37. Substrate 37 can be formed of anoptically transparent conductive material such as silicon carbide or canbe formed of an optically non-transparent, electrically conductivesubstrate such as copper or molybdenum. Such LEDs can be configured tohave either the n-layer, or p-layer in contact with the substrate.

In such LEDs, electrically conductive substrate 37 serves as oneelectrode while the other electrode 33 can be readily formed on the topsurface, e.g. p-layer 31. Since the contacts or electrodes are onopposing surfaces of LED 30, current flow is in a generally verticaldirection. Such devices are thus referred to as vertical structures.

Regardless of whether the metal electrodes are for vertical or lateralLED structures, they must satisfy similar requirements. Theserequirements include good adhesion, the ability to make ohmic contact tothe semiconductor, good electrical conductivity, and good reliability.Often, these requirements are satisfied by using two or more layers. Forexample a first layer of metal such as chromium or titanium can providegood adhesion and ohmic contact. A second layer of metal such as silveror gold can provide good electrical conductivity.

Although chromium has good adhesion and gold is a good electricalconductor. Neither material has good optical reflectivity in the visibleregion. The optical reflectivity and the corresponding opticalabsorption can be calculated from the refractive indices of thesestructures and their corresponding thicknesses.

Where a material thickness has not been given herein, the thickness canbe assumed to be great enough such that optical interference effects arenot an issue. For example, such reflectivity calculations typicallyassume the incident and exit medium to be semi-infinite. In cases ofmetal reflector layers where their thickness have not been specified,they are assumed to be thick enough, typically a few thousandnanometers, so that an insignificant amount of light reaches the othersurface of the metal. The refractive index values of Table 1 are used tocalculate all reflectivity curves in this disclosure. TABLE 1 RefractiveRefractive Dielectric Wavelength Index Index Material Abbreviation (nm)(Real) (Imaginary) Aluminum Al 450 0.49 −4.7 Titanium TiO₂ 450 2.57−0.0011 Dioxide Silicon SiO₂ 450 1.465 0 Dioxide Air Air 450 1 0 Gold Au450 1.4 −1.88 Chromium Cr 450 2.32 −3.14 Indium Tin ITO 450 2.116−0.0047 Oxide Titanium Ti 450 2.27 −3.04 Silver Ag 450 0.132 −2.72Gallium GaN 450 2.45 Nitride Nano Porous SiO₂_Nano 633 1.1 0 SiliconDioxide Titanium TiO₂ 633 2.67 0 Dioxide Gallium GaP 633 3.31 0Phosphide Silicon SiO₂ 633 1.456 0 Dioxide

The thickness of materials as referenced in this disclosure can be inabsolute units, T_(ABS), such as microns (μm) or nanometers (nm).Alternatively, the thickness of material can be given relative to thenumber of wavelengths in the medium, T_(1Rel). When given as the numberof wavelengths (λ), the parameter specifically refers to the wavelengthof light within the material itself. This can be converted to theabsolute thickness by multiplying by the index of refraction of thematerial (N) as indicated by Equation 1 below. For example a ¼λ of SiO₂at 450 nm would be 76.8 nm (0.25 450/1.465).T _(ABS)=(T _(λRel) /N)·λ  (Equation 1)

The optically reflectivity curve as a function of incident angle has twocomponents, i.e., P-polarized light and S-polarized light. P-polarizedlight experiences Brewster's angles and has a lower overall reflectivitythan S-polarized light.

Referring now to FIG. 4A, a diagram of a contemporary semiconductor andelectrode structure showing the reflectivity of an electrode 44 forlight originating within the semiconductor 41 is provided. The electrodeutilizes a typical chromium 42 and gold 43 electrode configuration andis formed upon a GaN semiconductor 41. For a reflection at an incidentangle of 45 degrees, an average of only 25% of the P-polarized andS-polarized light is reflected while, 75% of the light is absorbed.Thus, this contemporary configuration is undesirably highly absorbing.

Although FIG. 4A shows a gold/chromium metal electrode structure formedupon GaN, other metals and semiconductor materials can alternatively beutilized.

Referring now to FIG. 4B, a chart shows reflectivity at the GaN/Cr/Auinterface of the device of FIG. 4A for different angles of incidence.

Referring now to FIG. 5A, a more generic contemporary contact structureis shown. According to this more generic contact structure, there may bean ohmic contact and/or current spreading layer 52 between a metalcontact 53 and a semiconductor material 51. The metal contact 53 mayhave multiple layers for purposes for adhesion, diffusion barrier,solder, electrical conductivity, and ohmic contact. The layers can befabricated from various metals and combinations of metals, includingnickel, platinum, titanium, silver, aluminum, gold, tin, lead, andchromium. The semiconductor material 51 can be from the material systemssuch as AlGaAs, AllnGaP, AllnGaN, and GaAsP. The ohmic contact layer canbe part of the metal electrode layers such as nickel oxide.

Referring now to FIG. 5B, an electrically conductive metal oxide such asindium tin oxide or nickel oxide can be deposited on entire surface ofsemiconductor 55 to define an ohmic contact/current spreading layer 56upon which metal electrode 57 can be formed. In such a case, layer 56serves both as an ohmic contact and current spreading layer. There canbe a layer that allows for ohmic contact on the very top of the LEDsemiconductor material, such as a heavily doped region.

Regardless of the exact metal electrode configuration, semiconductormaterial or LED structure, contemporary metal electrodes undesirablyabsorb some light. In additional, metal contacts are not transparent,they block the available surface area where light can escape. Thus, suchcontemporary electrodes have a double effect. They not only directlyabsorb a portion of the incident light, but the remaining reflectedlight is directed back into the device where it suffers furtherabsorption by other materials. The total amount of absorption is highlydependent on the exact configuration of the electrode and tends to scaleproportionally to the size of the electrode contact area.

Referring now to FIGS. 6A-6C, the principle of current spreading so asto mitigate the problem of current crowding is discussed. The p-layerand n-layer of contemporary LEDs are thin and have relatively lowelectrical conductivity. By themselves, these layers do not evenlydistribute current to all regions of the p-n junction, i.e., the activeregion. For larger areas where portions of the active region are faraway from the electrode, there will be less current flow in thesedistant areas than in areas close to the metal contact. This results inuneven current distribution and consequent uneven light emission. Toreduce current crowding, the geometry of the metal electrodes isextended over the semiconductor surface. These extensions however leadto additional undesirable light absorption.

With particular reference to FIG. 6A, a circular contact or electrode 62can be formed upon a semiconductor 61 and can serve as a wire bond pad.With particular reference to FIG. 6B, a cross shaped contact 63 can becombined with electrode 62 to enhance current spreading. With particularreference to FIG. 6C, various other geometrical structures 63 cansimilarly be combined with electrode 62 to facilitate current spreading,especially on larger LED dies.

Typically, wire bonds are used as a means to provide electric power theLED. However the wire bond pad areas must be some minimum size of about100 μm by 100 μm. Since the size of each wire bond pad is fixedregardless of device size, the absorbing and opaque wire bond areas canbe a significant portion of the overall surface area and for same LEDdevices.

One method for reducing the undesirable absorption of light by anelectrode is to minimize the contact area or the width of the electrode.If electrical connection to the LED semiconductor material is the onlyconsideration, then the contact width can be quite narrow, such as onthe order of a few microns. However, an important consideration is theundesirable increase of electrical resistivity caused by decreasing thecross sectional area. In high power applications, the electrode maycarry an amp or more of current. This requires the cross sectional area,width (W)×thickness (T) to be of some minimum value to minimizeelectrical resistance. Thus, the contact area or width of the electrodecannot merely be reduced without otherwise compensating for the increasein resistivity of the electrode.

Referring now to FIG. 7A, a typical dimension for a gold electrode isW=20 μm and T=2 μm for a total cross sectional area of 40 μm².Theoretically, one could keep a constant cross sectional area andtherefore a constant electrical resistance by proportionally increasingthickness while decreasing the width as discussed with reference to FIG.7B below.

Referring now to FIG. 7B, according to one embodiment of the presentinvention the aspect ratio of electrode 77 can be increased. That is,the height of electrode 77 can be increase as compared to the widththereof. For example, the height can be increase so as to provide athickness greater than 2.5 μm. In this manner, the area of electrode 74that is in contact with semiconductor 75 (and is thus available forlight absorption) is reduced and light absorption is consequentlysimilarly reduced. Increasing the height of electrode 77 desirablymaintains its conductivity. The contact area has been decreased and thethickness of the electrode has been increased so as to maintain desiredconductivity. However manufacturing cost and practical processconsiderations typically limit electrode thickness to 2.5 μm or below.Thus the electrode contact area and its associated absorption becomemuch greater than would be necessary if the electrode was used for onlyelectrical contact to the semiconductor material.

Another method for reducing electrode absorption is to increase thereflectivity of the electrode. Several prior art approaches have beenused to create reflective electrodes for LEDs. The simplest is to use ametal that has a high reflectivity. These include Al, Ag, Re and othersknown to one familiar with the art.

The chosen metal needs to not only have a high reflectance, but mustalso make an acceptably low resistance ohmic contact to thesemiconductor material. In the case of p-type AllnGaN, only Ag combineslow electrical resistance with high reflectivity.

Referring now to FIG. 8A, an electrode structure comprised of Ag isshown. That is, an Ag electrode 82 is formed upon a semiconductorsubstrate 81. Unfortunately, Ag presents a reliability concern becauseit is subject to tarnish and it is subject to electromigration duringdevice operation. Also, the contact resistance of Ag-based contactssometimes increases with time during device operation.

Referring now to FIG. 8B, the reflectance of the Ag electrode of FIG. 8Afor different angles of incidence is shown. Even with a highlyreflective metal electrode, silver, the absorption per reflection nearnormal incidence is about 10%. It would be desirable to further decreaseabsorption to well below 10%.

Referring now to FIG. 9A, it is known to use a ¼λ layer of dielectric103, i.e., SiO₂, to enhance reflectivity in a vertical structure LED.The dielectric 103 is formed between a GaN semiconductor 104 and an Agmetal layer 102, both of which are formed upon a conductive holder 101.However, as discussed below, the use of a ¼λ of dielectric does notsubstantially enhance reflectivity.

Referring now to FIG. 9B, it can be seen that the use of the ¼λ layer ofdielectric does provide enhanced reflectance for the S polarized lightincident thereon, as indicated by curve 153. However, the P polarizedlight incident upon this dielectric layer has a deep dip in thereflectance curve around 47°, as indicated by curve 152. This dipsubstantially reduces the overall reflectivity, as indicated by thecurve 151 for the average of the S polarized and the P polarized light.Therefore, the use of a ¼λ layer of dielectric is not a suitablesolution to the problem of light absorption by an LED electrode.

According to one embodiment of the present invention, a reflectiveelectrode structure minimizes contact area between the electrode and theLED semiconductor material. A comparatively thick dielectric material isdisposed between a conductive electrode and the semiconductor materialso as to electrically isolate portions of the electrode while allowingfor other portions to make electrical contact. The dielectric materialcan be of a lower index of refraction than the semiconductor and can bethick enough such that total internal reflection occurs for incidentangles greater than the critical angle Θ_(c), as discussed below.

Total internal reflection for dielectric materials provides thedesirable capability for approximately 100% reflectivity. Total internalreflection occurs beyond the critical angle, Θ_(c). In the case of a GaNto air interface, the critical angle is approximately 24°. In the caseof a GaN to SiO₂ interface, the critical angle is approximately 37°.

Referring now to FIG. 10A, a semi-schematic diagram shows lightreflection at a GaN/air. A ray of light is shown being reflected fromthe interface back into the GaN semiconductor material 121 because theangle of incidence is greater than the critical angle Θ_(c).

Referring now to FIG. 10B, a chart shows reflectivity at the GaN/airinterface of FIG. 10A for different angles of incidence.

Referring now to FIG. 11A, a semi-schematic diagram shows lightreflection at a GaN/SiO₂ interface according to an embodiment of thepresent invention. A ray of light is shown being reflected from theinterface of the GaN semiconductor material 131 and the SiO₂ layer 132back into the GaN semiconductor material 131 because the angle ofincidence is greater than the critical angle Θ_(c).

Referring now to FIG. 11B, a chart shows reflectivity at the GaN/SiO₂interface of FIG. 11A for different angles of incidence according to anembodiment of the present invention.

Referring now to FIG. 12A, is a semi-schematic diagram show lightreflection at a GaN/SiO₂/Al interface according to an embodiment of thepresent invention. A portion of electrode 173 is suspended over GaNsubstrate 171 and has a thick dielectric SiO₂ layer 172 formedtherebetween. Another portion of electrode 173 is formed directly uponGaN substrate 171.

Referring now to FIG. 12B, is a chart showing the P-polarizationreflectivity at the GaN/SiO₂/Al interface of FIG. 12A for differentangles of incidence wherein thicknesses of the SiO₂ layer are less thanor equal to 1¾λ according to an embodiment of the present invention. Ata 1/16λ of SiO₂ there is no total internal reflection effect and thereflectivity is marginally worse than without the SiO₂ layer. At a ¼λ ofSiO₂ there is still no TIR effect and the reflectivity is dramaticallyworse. At ½λ of SiO₂ total internal reflection does occur for largeincident angles but a tremendous dip in reflectivity occurs atapproximately 38°. At 1¾λ, total internal reflection occurs for the highangles of incidence and no noticeable dip in reflectivity. Since TIRbegins at ½λ of SiO₂, the term “thick” dielectric will refer to alldielectrics thicker or equal to ½λ.

Referring now to FIG. 12C, is a chart showing reflectivity at theGaN/SiO₂/Al interface of FIG. 12A for different angles of incidencewherein thicknesses of the SiO₂ layer are greater than 1¾the wavelengthof incident light according to an embodiment of the present invention.

Once the dielectric layer is greater than this minimum thickness fortotal internal reflection, its exact thickness is not as critical as inconventional optical coatings based on interference. This allows forgreater latitude in the manufacturing process. This is illustrated inFIG. 12C, which shows the reflectivity curves of for a thick dielectricat two different thicknesses, one at 1.75 λ, and the other at 1.85 λ.The total internal reflection angle does not change.

Referring now to FIG. 13A, a semi-schematic diagram shows lightreflection at a distributed Bragg reflector (DBR) comprised ofalternating layers of SiO₂ 182 and TiO₂ 183 on top of the thickdielectric SiO₂ base layer 185 according to an embodiment of the presentinvention. An electrode 184 makes electrical contact to semiconductormaterial 181 and is the final layer onto top of the DBR stack. Thickdielectric layer 185 is formed between the DBR stack and semiconductormaterial 181.

The thick dielectric creates an effective reflector at high angles.However, it does not substantially enhance the reflectivity below thecritical angle. It is possible to add a distributed Bragg reflector(DBR) to reflect the light at these lower angles. DBRs are typicallyfabricated using a series of alternating high index/low index dielectricmaterials. As shown in FIG. 13A, a series of 2 pairs of ¼λ SiO₂ and ¼λTiO₂ over a thick layer of 1 ¾λ SiO₂ enhance the reflectivity at lowerangles. DBRs use optical interference to affect reflectivity, as resulttheir thickness is more critical than the thickness of the underlyingthick SiO₂ layer.

Table 2 below provides further information regarding the electrodematerials utilized according to one or more embodiments of the presentinvention. The reference wavelength for the coating thickness is 0.4500microns. The phase and retardance values are in degrees. The coating hassix layers. The incident media is GaN. The wavelength of the light usedis 0.4500 microns. TABLE 2 Material Thickness Al₁ 1.000000 SiO₂ 0.250000TiO₂ 0.250000 SiO₂ 0.250000 TiO₂ 0.250000 SiO₂ 0.750000

Referring now to FIG. 13B, is a chart showing reflectivity at the DBRlayers of FIG. 13A for different angles of incidence according to anembodiment of the present invention compared to a design with only thickdielectric compared to a design with no thick dielectric and no DBR.

Referring now to FIG. 14, is a chart showing reflectivity of severalmaterials for different angles of incidence according to an embodimentof the present invention as compared to prior art. A Au metal layer witha Cr under layer has the worst reflectance as indicated by the lowestcurve 1951. Al is substantially better as indicated by curve 1952. Ag iseven better as indicated by curve 1953. An Ag metal layer with a thickSiO₂ dielectric under layer has generally better reflectance than Ag,although curve 1954 dips below curve 1953 in some places. An Ag metallayer with 2 pairs of DBR followed by with a thick SiO₂ has the bestreflectance, as indicated by curve 1955.

Referring now to FIG. 15A, a semi-schematic diagram shows a firstexemplary embodiment of a suspended electrode according to the presentinvention. Electrode 142 a is suspended above a GaN substrate 141 suchthat a thick air gap 143 a is formed therebetween. Electrode 142 a issupported on both sides thereof.

Referring now to FIG. 15B, a semi-schematic diagram shows a secondexemplary embodiment of a suspended electrode according to the presentinvention. Electrode 142 b is suspended above the GaN substrate 141 suchthat a plurality of air gaps 143 b are formed therebetween. Electrode142 a is supported on both sides and in the middle thereof.

Referring now to FIG. 15C, a semi-schematic diagram shows a thirdexemplary embodiment of a suspended electrode according to the presentinvention. Electrode 142 c is suspended above the GaN substrate 141 suchthat a thick air gap 143 c is formed therebetween. Electrode 142 c issupported only on one side thereof.

Referring now to FIG. 15D, a semi-schematic diagram shows a fourthexemplary embodiment of a suspended electrode according to the presentinvention. Electrode 142 d is suspended above the GaN substrate 141 anda thick SiO₂ layer 143 d is formed therebetween. Electrode 142 d issupported on both sides thereof.

Referring now to FIG. 15E, a semi-schematic diagram shows a fifthexemplary embodiment of a suspended electrode according to the presentinvention. Electrode 142 e is suspended above the GaN substrate 141 anda plurality of sections of a thick SiO₂ layer 143 e are formedtherebetween. Electrode 142 e is supported on both sides and in themiddle thereof.

Referring now to FIG. 15F, a semi-schematic diagram shows a sixthexemplary embodiment of a suspended electrode according to the presentinvention. Electrode 142 f is suspended above the GaN substrate 141 suchthat a thick SiO₂ layer 143 f is formed therebetween. Electrode 142 f issupported only on one side thereof.

Referring now to FIG. 16A, a semi-schematic diagram shows a firstexemplary embodiment of a suspended electrode with an ohmic contactlayer according to the present invention. The structure of the electrodeof FIG. 16A is similar to that of FIG. 15A, except for the addition ofindium tin oxide (ITO) layer 144.

Referring now to FIG. 16B, a semi-schematic diagram shows a secondexemplary embodiment of a suspended electrode with an ohmic contactlayer according to the present invention. The structure of the electrodeof FIG. 16B is similar to that of FIG. 16B, except for the addition ofindium tin oxide (ITO) layer 144.

Referring now to FIG. 16C, a semi-schematic diagram shows a thirdexemplary embodiment of a suspended electrode with an ohmic contactlayer according to the present invention. The structure of the electrodeof FIG. 16C is similar to that of FIG. 15C, except for the addition ofindium tin oxide (ITO) layer 144.

Referring now to FIG. 16D, a semi-schematic diagram shows a fourthexemplary embodiment of a suspended electrode with an ohmic contactlayer according to the present invention. The structure of the electrodeof FIG. 16D is similar to that of FIG. 15D, except for the addition ofindium tin oxide (ITO) layer 144.

Referring now to FIG. 16E, a semi-schematic diagram shows a fifthexemplary embodiment of a suspended electrode with an ohmic contactlayer according to the present invention. The structure of the electrodeof FIG. 16E is similar to that of FIG. 15E, except for the addition ofindium tin oxide (ITO) layer 144.

Referring now to FIG. 16F, a semi-schematic diagram shows a sixthexemplary embodiment of a suspended electrode with an ohmic contactlayer according to the present invention. The structure of the electrodeof FIG. 15F is similar to that of FIG. 14F, except for the addition ofindium tin oxide (ITO) layer 144.

Referring now to FIGS. 17A-17D, an exemplary, contemporary, lateral LEDstructure and the process for forming it are shown.

With particular reference to FIG. 17A, a pair of wire bond pads 1091 and1092 facilitate the application of current to a semiconductor 1093.Semiconductor 1093 is formed upon a substrate 1096. Semiconductor 1093comprises an p-layer 1097 and a n-layer 1098 (n-layer 1098 and p-layer1097 are generally interchangeable for the purposes of this discussion)The current causes active region 1094 to produce light according to wellknown principles.

With particular reference to FIG. 17B, the fabrication of the LED ofFIG. 9A comprises forming a semiconductor layer 1093 upon a substrate1096. Semiconductor layer 1093 comprises an n-layer 1098 and a p-layer1097 (as shown in FIG. 17A).

With particular reference to FIG. 17C, a portion of p-layer 1097 isremoved, such as by etching. A sufficient amount of p-layer 1097 isremoved so as to expose a portion of n-layer 1098 therebeneath. Removalof the portion of p-layer 1097 defines a cutout portion 1099. Theformation of cut out 1099 leaves n-layer 1098 exposed.

With particular reference to FIG. 17D, wire bond pad 1091 is formed uponp-layer 1097 and wire bond pad 1092 is formed upon n-layer 1098, wirebond pads 1091 and 1092 cover a comparatively large portion of thesurface area of semiconductor 1093. For example, the electrode wire bondpads of a contemporary LED can be 100 μm×100 μm. They thus absorb anundesirably large amount of the light produced by active region 1094.Further, the comparatively large cut out area 1099 that is required forwire bond pads 1092 undesirably reduces the size of active area 1094 andthus further reduces the amount of light produced by such contemporaryLEDs. Since the size of each electrode is fixed regardless of devicesize, the undesirable light absorption can be a significant portion ofthe overall surface area, particularly for smaller LEDs.

It is worthwhile to appreciate that the formation of such an electrodestructure that is partially within and partially outside of a cutoutoffers substantial advantage, even if the electrode is not reflective.For example, the electrode structure described in connection with FIGS.18A-18B below provides adequate bonding area while minimizing the sizeof the cutout such that less active area is removed and more light canbe produced.

Referring now to FIGS. 18A-18E, an exemplary lateral LED structure andthe process for forming it according to an embodiment of the presentinvention are shown. A thick dielectric layer 1101 and 1102 is formedbeneath wire bond pads 1091 a and 1092 a, respectively. Thick dielectriclayers 1101 and 1102 enhance the reflectivity of wire bond pads 1091 aand 1092 a such that undesirable light absorption thereby issubstantially decreased. A portion of each wire bond pad 1091 a and 1092a remains in contact with semiconductor 1093 so as to facilitate currentflow therethrough.

As used herein, a thick dielectric layer is a layer having sufficientthickness such that effects of interference are not substantial.Moreover, as used herein a thick dielectric layer can have a thicknessof greater than ¼λ. For example, a thick dielectric layer can have athickness equal or great then ½λ, approximately 1.5 λ, approximately1.75 λ, or greater than 1.75λ.

With particular reference to FIGS. 18B and 18C, semiconductor 1093 isformed upon substrate 1096 and cutout 1099 is formed in semiconductor1093 as in FIGS. 17B and 17C.

With particular reference to FIG. 18D, thick dielectric layers 1101 and1102 are formed upon p-layer 1097 and n-layer 1098, respectively. Thickdielectric layers 1101 and 1102 can be formed according to well knownprinciples.

With particular reference to FIG. 18E, wire bond pad 1091 a is formed soas to at least partially cover thick dielectric layer 1101 and wire bondpad 1092 a is formed so as to at least partially cover thick dielectriclayer 1102. As mentioned above, a portion of wire bond pads 1091 a and1092 a contacts semiconductor 1093 therebeneath.

Referring now to FIG. 19A-19E an exemplary lateral LED structure and theprocess for forming it according to an embodiment of the presentinvention are shown.

With particular reference to FIGS. 19A, a thick dielectric layer 1101and 1102 a is formed beneath wire bond pads 1091 a and 1092 b,respectively. Thick dielectric layers 1101 and 1102 a enhance thereflectivity wire bond pads 1091 a and 1092 b such that undesirablelight absorption thereby is substantially decreased. A portion of eachwire bond pad 1091 a and 1092 b remains in contact with semiconductor1093 so as to facilitate current flow.

With particular reference to FIGS. 19B and 19C, semiconductor 1093 isformed upon substrate 1096 and cutout 1099 a is formed in semiconductor1093 as in FIGS. 17B and 17C. However, in this embodiment cutout 1099 ais formed in an L-shaped configuration so as to mitigate the amount ofsurface area thereof. In this manner, less of the active area issacrificed in the formation of cutout 1099 a and the brightness of theLED is consequently enhanced.

With particular reference to FIG. 19D, a thick dielectric layer 1101 isformed upon the p-layer 1097. Another thick dielectric layer 1102 a isformed partially on the player 1097 and partially on the n-layer 1098.Thick dielectric layers 1101 and 1102 a can again be formed according towell known principles. In this instance thick dielectric layer 1102 a isformed downwardly, along the side of p-layer 1097 and active layer 1094so as to electrically insulate wire bond pad 1092 b therefrom. That is,thick dielectric layer 1102 a is formed upon both p-layer 1097 andn-layer 1098, as well as the interface therebetween, i.e., active layer1094. Thick dielectric layer 1102 a stair steps downwardly from n-layer1097 to n-layer 1098. This configuration of thick dielectric layer 1102a is best seen in the cross section of FIG. 19A.

With particular reference to FIG. 19E, wire bond pad 1091 a is formed soas to at least partially cover thick dielectric layer 1101 and wire bondpad 1092 b is formed so as to at least partially cover thick dielectriclayer 1102 a. As mentioned above, a portion of wire bond pad 1091 acontacts p-layer 1097 and a portion of wire bond pad 1092 b contactsn-layer 1098. In this instance, wire bond pad 1092 b is formeddownwardly, insulated by and covering thick dielectric layer 1102 a andelectrically contacting n-layer 1098. The configuration of wire bond pad1092 b is best seen in FIG. 19A.

In this embodiment, thick dielectric layers 1101 and 1102 asubstantially mitigate light absorption by wire bond pads 1091 a and1092 b so as to enhance the brightness of the LED. The reduced size ofcutout 1099 a provides a larger active area 1094, so as to furtherenhance the brightness of the LED.

According to the present invention, a thick dielectric can be formedbetween at least a portion of each bond pad and/or electrode and thesemiconductor material. The thick dielectric material enhancesreflectivity such that undesirable light absorption by the bond padand/or electrode is substantially mitigated.

Referring now to FIG. 20A, a semi-schematic perspective view shows oneembodiment of a suspended electrode structure according to an embodimentof the present invention. A metal electrode 162 is formed upon asemiconductor 161. A thick dielectric 163 is formed between metalelectrode 162 and semiconductor 161. A portion of electrode 162 isformed over thick dielectric 163 and a portion of electrode 162 contactssemiconductor 161 such that electrode 162 is in electrical contact withsemiconductor 161.

Referring now to FIG. 20B, a semi-schematic perspective view showsanother configuration of a suspended electrode structure according to anembodiment of the present invention. This structure is generally similarto that of FIG. 20A except that thick dielectric 163 is broken up suchthat portions of electrode 162 contact semiconductor is different placesthan in FIG. 20A. As shown in FIG. 20B, multiple contacts of electrode162 to semiconductor 161 are provided. As those skilled in the art willappreciate, various configurations of electrode 162 and thick dielectric163, with electrode 162 contacting semiconductor 161 in variousdifferent places, are possible.

FIGS. 21A-24 show exemplary electrode structures that utilize thickdielectrics according to one or more embodiments of the presentinvention. For example, one or more layers of insulating dielectric canbe formed under the bonds pads. Some advantages of such constructioninclude: the mitigation of current crowding, thus facilitating asimplified design; the minimization of light absorption because thedielectric layer(s) under the electrode can form a mirror; moreefficient use of the emission area that is achieved by reducing thecutout area; a more easily scalable design for a large range of diesizes; comparatively low forward voltage; and more even currentspreading.

The exemplary embodiments of FIGS. 21A-24 are implementations of anelongated chip. Such elongated chips can provide enhanced brightnesswith better efficiency.

Referring now to FIG. 21A, an electrode design for an elongated chip isshown. Thick dielectric layers 1002 and 1003 can be formed under each ofthe bond pads 1006 (the p-bond pad, for example) and 1007 (the n-bondpad, for example). N-bond pad 1007 and n-electrode extension 1001 areformed upon an etched away portion of semiconductor material 1008 orcutout 1004

The thick dielectric layers 1002 and 1003 insulate the bond pads 1006and 1007 from semiconductor material 1008 so as to mitigate currentcrowding. This results in an improved geometry for more even currentflow. Hot spots that cause uneven brightness and can result in damage tothe LED are substantially mitigated.

Such thick dielectric layers are not formed under conductive extensions1001 and 1005 that define n-wiring and p-wiring respectively. Extensions1001 and 1005 thus more evenly distribute current throughoutsemiconductor 1008. That is, the distance between the electrodes thatprovide current to the LED tends to be more equal according to oneaspect of the present invention.

It is worthwhile to appreciate that total internal reflection (TIR)provides a substantial advantage in enhancing light extraction for oneor more embodiments of the present invention. The use of a DBR structureis optional and can be used, according to at least one embodiment of thepresent invention, to further enhance light extraction.

The use of TIR and/or DBR structures as described above cansubstantially mitigate undesirable absorption of light under bond pads1006 and 1007. Such insulators (as well as insulating layers 1002 and1003) can be formed beneath bond pads 1006 and 1007 and not beneathextensions 1001 and 1005, so that current flow through semiconductor(and consequently the active region thereof) is more evenly distributed.

Bond pads 1006 and 1007, as shown in FIGS. 21A and 21B, are not locatedexactly at the end of the wire traces or extensions 1001 and 1005. Thisis to show that bond pads 1006 and 1007 can be placed at any arbitrarylocation along the trace. Thus, bond pads 1006 and 1007 can be placed atthe end, near the end, and/or in the middle of extensions 1001 and 1005.Any desired location of bond pads 1006 and 1007 can be used.

Referring now to FIG. 21B, a potential improvement with respect to theconfiguration of FIG. 21A is shown. The area of cutout 1104 is reducedby putting the n-bond pad above the p-surface and separated form thep-surface by the thick dielectric. That is, at least a portion of then-bond pad is not in cutout 1104 and cutout 1104 can thus be muchsmaller than in FIG. 21A. This thick dielectric must also cover theedges of the cutout to ensure isolation of the n-bond pad from thep-Layer. That is, the area of the cutout is reduced such that the sizeof the active area is increased. The larger emission area facilitated byusing a smaller cutout 1004 can enable a greater power output.

In some applications, the distance between the p and n electrodes may betoo great, thus resulting in an undesirably high forward voltage. Insuch cases, the use of multiple electrodes may be beneficial. FIGS.22A-23C show various exemplary implementations of three electrodedesigns that can mitigate such undesirability high forward voltages.

Referring now to FIGS. 22A-22C, the n-bond pad is shown split into twoelectrically isolated pads 1217 and 1218. In principle, they can betouching (and thus in electrical contact with one another) and thuseffectively form a single pad. There can be two separate wire bonds, oneto each of pads 1217 and 1218. However if a gap 1220 between pad 1217and 1218 is small enough, then a single bond pad can be used toelectrically connect bond pads 1217 and 1218 together. In this manner,any desired number of such electrodes can be used.

With particular reference to FIG. 22A, two n-bond pads 1217 and 1218 anda single p-bond pad 1219 can be used. Two thick dielectric layers 1204and 1283 can be formed between each bond pad 1219 and the semiconductormaterial 1280 disposed therebeneath. Similarly, a thick dielectric layer1202 can be formed between bond pads 1217 and 1218 and the semiconductormaterial 1201 of cutout 1281. As mentioned above, such constructionresults in more even current distribution. This is particularly true forlarger and/or higher current LEDs.

With particular reference to FIG. 22B, the area of cutout 1201 isreduced with respect to that shown in FIG. 22A in a manner analogous tothat of FIG. 21B. Again, two thick dielectric layers 1204 and 1283 canbe formed between each bond pad 1219 and the semiconductor material 1280disposed therebeneath. Similarly, a thick dielectric layer 1202 can beformed between bond pads 1217 and 1218 and the semiconductor material1201 of cutout 1281.

With particular reference to FIG. 22C, p-wiring extension 1203 extendsbeneath n-bond pad thick dielectric 1202 such that a distal end 1230 ofp-wiring extension extends to the right of thick dielectric 1202. Again,two thick dielectric layers 1204 and 1283 can be formed between eachbond pad 1219 and the semiconductor material 1280 disposed therebeneath.Similarly, a thick dielectric layer 1202 can be formed between bond pads1217 and 1218 and the semiconductor material 1201 of cutout 1281.

With particular reference to FIG. 23A-23C, the p-layer and the n-layerare reversed in position (with a consequent reversal in the respectivebond pads, insulators, etc) to show that the construction of FIGS.22A-22C is suitable with either type of device. Thus, n-bond 1507 andthick n-bond pad dielectric 1503 are formed on cutout 1504 and p-bondpads 1511 and 1512 and thick p-bond pad dielectric 1501 are not formedon cutout 1504 (which is the opposite of the construction shown in FIGS.22A-22C). Thus, the electrodes are reversed with respect to those shownin FIGS. 22A-22C.

With particular reference to FIG. 24, a two electrode LED thatfacilitates more uniform current distribution is shown. An n-bond pad2403 and a p-bond pad 2404 are formed upon a semiconductor material2401. n-bond pad 2403 has a thick dielectric layer 2406 form betweenitself and semiconductor material 2401. Similarly, p-bond pad 2404 has athick dielectric layer 2407 formed between itself and semiconductormaterial 2401.

A cutout 2402 facilitates contact of n-bond pad 2403 to the n-layer ofsemiconductor 2401. A portion of n-bond pad 2403 can be formed outsideof cutout 2402 (and thus upon the p-layer of semiconductor material2401) and a portion of n-bond pad 2403 can be formed within cutout 2402(to provide electrical contact with the n-layer). Similarly, a portionof thick dielectric layer 2406 can be formed outside of cutout 2402 (andthus upon the p-layer of semiconductor material 2401) and a portion ofthick dielectric layer 2406 can be formed within cutout 2402.

n-bond pad 2403 and thick dielectric layer 2406 thus extend down theside of cutout 2402 from the n-layer to the p-layer of semiconductormaterial 2401, in a fashion similar to that of FIG. 21B. Suchconstruction tends to minimize the size of cutout 2402 and thus tends toenhance the brightness and efficiency of the LED, as discussed above.

p-wiring or extension 2407 extends from p-pad 2404 so as to moreuniformly distribute current through the active region of semiconductor2401. A portion of p-pad 2404 and all of extension 2407 can be formeddirectly upon semiconductor material 2401 (without a thick dielectriclayer therebetween).

Although in FIGS. 15-24 only a single thick dielectric layer is shown, aseries of one or more DBR pairs can be deposed between the thickdielectric and the electrode. Similarly, although FIGS. 15-24 show theelectrode in direct contact with the semiconductor material, the contactcan be via an ohmic contact layer or current spreading layer.

According to one or more embodiments of the present invention, the thickdielectric can be non-perforated. That is, the dielectric can becontinuous in cross-section. It can be formed such that it does not haveany holes or perforations that would cause the thick dielectric toappear to be discontinuous in cross-section.

The dielectric material can be porous. Thus, thick dielectric materialswhich may otherwise be too dense (and thus have to high of an index ofrefraction) can be used by effectively reducing the density (and theeffective index of refraction, as well) by making the dielectricmaterial porous or non continuous.

In view of the foregoing, one or more embodiments of the presentinvention provide a brighter and/or more efficient LED. Increasing thebrightness of an LED enhances its utility by making it better suited foruse in a wide of applications. For example, brighter LEDs can besuitable for general illumination applications. Further, more efficientLEDs are desirable because they tend to reduce the cost of use (such asby reducing the amount of electricity required in order to provide adesire amount of light.

Embodiments described above illustrate, but do not limit, the invention.It should also be understood that numerous modifications and variationsare possible in accordance with the principles of the present invention.Accordingly, the scope of the invention is defined only by the followingclaims.

1. An electrode structure for a semiconductor LED, the electrodestructure comprising: a metal electrode; and an optically transmissivedielectric material having an index of refraction greater than or equalto one and less than that of a semiconductor upon which the dielectricmaterial is formed and having a thickness greater than ½λ, thedielectric material being formed to the electrode so as to enhancereflection of light.
 2. The electrode structure as recited in claim 1,wherein: a portion of the electrode is in electrical communication witha semiconducting material; and another portion of electrode is deposedon top of the dielectric material.
 3. The electrode structure as recitedin claim 1, wherein the dielectric material has a thickness ofapproximately 1.75λ.
 4. The electrode structure as recited in claim 1,wherein: the electrode is formed upon a semiconductor material; and thedielectric material is formed intermediate a portion of the electrodeand the semiconductor material.
 5. The electrode structure as recited inclaim 1, wherein: the electrode is formed upon a semiconductor material;the dielectric material is formed intermediate at least a portion of theelectrode and the semiconductor material; and the semiconductor materialcomprises at least one material selected from the group consisting of:AlGaAs; AllnGaP; AllnGaN; and GaAsP.
 6. The electrode structure asrecited in claim 1, wherein the dielectric material comprises at leastone material selected from the group consisting of: silicon dioxide;silicon monoxide; MgF₂; siloxane polymers; and air.
 7. The electrodestructure as recited in claim 1, further comprising an ohmic contactlayer formed between the electrode and a semiconductor.
 8. The electrodestructure as recited in claim 1, further comprising: an ohmic contactlayer formed between the electrode and a semiconductor; and wherein theohmic contact layer comprises at least one material selected from thegroup consisting of: indium tin oxide; nickel oxide; and RuO₂.
 9. Theelectrode structure as recited in claim 1, further comprising: an ohmiccontact layer formed between the electrode and a semiconductor; whereinthe ohmic contact layer is part of the semiconductor device; and whereinthe ohmic contact layer comprises a heavily doped layer.
 10. Theelectrode structure as recited in claim 1, further comprising a currentspreading layer formed between the metal and a semiconductor.
 11. Theelectrode structure as recited in claim 1, further comprising: a currentspreading layer formed between the metal and a semiconductor; andwherein the current spreading layer comprises at least one materialselected from the group consisting of: indium tin oxide; nickel; andRuO₂.
 12. The electrode structure as recited in claim 1, furthercomprising: at least one pair of dielectric layers configured so as todefine a DBR structure and disposed between the electrode and thedielectric; and wherein each pair of dielectric layers is substantiallyoptically transmissive, is comprised of layers of materials of differentindices of refraction, and is a multiple of approximately ¼λ thick. 13.The electrode structure as recited in claim 1, further comprising: adielectric base layer; at least one pair of DBR dielectric layers formedbetween the electrode and the dielectric base layer, the dielectric baselayer having a thickness greater than ½λ; and wherein each DBRdielectric layer pair comprises at least one material selected from thegroup consisting of: TiO₂; Ti₃O₅; Ti₂O₃; TiO; ZrO₂; TiO₂ZrO₂Nb₂O₅; CeO₂;ZnS; A120 ₃; SiN; ITO; niobium pentoxide (Nb₂ 0 ₅); tantalum pentoxide(Ta₂ 0 ₅); and siloxane polymers SiO, SiO₂, or MgF₂.
 14. The electrodestructure as recited in claim 1, wherein the electrode comprises atleast one metal layer.
 15. The electrode structure as recited in claim1, wherein; the electrode comprises at least one metal layer; andwherein the metal layer(s) are selected from the group consisting of:Al; Ag; Rh; Pd; Cu; Au; Cr; platinum; titanium; nickel/gold alloys;chrome/gold alloys; silver/aluminum mixtures; and combinations thereof.16. The electrode structure as recited in claim 1, wherein the electrodeis formed upon a semiconductor material so as to define a lateralstructure.
 17. The electrode structure as recited in claim 1, whereinthe electrode is formed upon a semiconductor material so as to define avertical structure.
 18. The electrode structure as recited in claim 1,wherein a portion of the metal forms an area for wire bonding.
 19. Theelectrode structure as recited in claim 1, wherein a portion of themetal makes an electrical contact to a semiconductor material at theedges of a dielectric material.
 20. The electrode structure as recitedin claim 1, wherein a portion of the metal makes an electrical contactto the semiconductor material through openings in the dielectricmaterial.
 21. The electrode structure as recited in claim 1, wherein thedielectric material is porous.
 22. The electrode structure as recited inclaim 1, wherein the dielectric material comprises porous ITO.
 23. Theelectrode structure as recited in claim 1, wherein the dielectricmaterial is porous SiO₂.
 24. The electrode structure as recited in claim1, wherein the dielectric material is porous enough to reduce itseffective index of refraction such that total internal reflection occurswithin the semiconductor material.
 25. A reflective electrode structurecomprising: a metal electrode; an AllnGaN material configured to emitlight about a central wavelength λ; silicon dioxide dielectric materialformed upon the AlInGaN material, the dielectric material having athickness greater than approximately ½λ; and wherein a portion of theelectrode is formed over the dielectric material and another portion ofthe electrode is in ohmic contact with a semiconductor material.
 26. Thereflective electrode structure as recited in claim 27, wherein thedielectric material and electrode make physical contact to thesemiconductor via an ITO layer.
 27. A reflective electrode structurecomprising: a metal electrode; a GaN material configured to emit lightabout a central wavelength λ; silicon dioxide dielectric material formedupon the GaN material, the dielectric material having a thickness ofapproximately 1.75λ; at least one DBR pair formed upon the silicondioxide material; and wherein a portion of the electrode is formed overboth the dielectric material and the DBR pair(s) and another portion ofthe electrode is in ohmic contact with the semiconductor material. 28.The reflective electrode structure as recited in claim 29, wherein eachlayer of the DBR pair(s) is optically transmissive, of different indicesof refraction with respect to each other layer, and have a thicknessthat is a multiple of approximately ¼λ.
 29. The reflective electrodestructure as recited in claim 29 wherein the dielectric and theelectrode make physical contact to the semiconductor via an ITO layer.30. A method for forming an electrode structure for an LED, the methodcomprising forming a dielectric material upon a semiconductor materialand forming a conductive electrode such that the dielectric materialelectrically isolates at least one portion of the electrode from thesemiconductor material while allowing at least one other portion of theelectrode to make electrical contact with the semiconductor material.31. An electrode structure for an LED, the electrode structurecomprising: a semiconductor material having an n-portion and ap-portion; a metal n-electrode in electrical communication with then-portion; a metal p-electrode in electrical communication with thep-portion; an optically transmissive dielectric material formed to atleast one of the n-electrode and the p-electrode so as to enhancereflection of light, the electrode having a thickness of approximately1.75λ; and wherein the semiconductor material, the n-electrode and thep-electrode cooperate to define a lateral LED structure.